Re: pre Sandy bridge IOMMU support (gm45)

[Thierry Laurion <thierry.laurion@gmail.com>]

[-- Attachment #1.1: Type: text/plain, Size: 3784 bytes --]

Le lun. 1 févr. 2016 à 07:35, Jan Beulich <JBeulich@suse.com> a écrit :

> >>> On 01.02.16 at 13:28, <marmarek@invisiblethingslab.com> wrote:
> > On Mon, Feb 01, 2016 at 12:59:00AM -0700, Jan Beulich wrote:
> >> >>> On 30.01.16 at 02:47, <marmarek@invisiblethingslab.com> wrote:
> >> > On Tue, Jan 26, 2016 at 04:37:05AM -0700, Jan Beulich wrote:
> >> >> (re-adding xen-devel)
> >> >>
> >> >> >>> On 26.01.16 at 12:28, <thierry.laurion@gmail.com> wrote:
> >> >> > Iommu=0 let the whole Qubes system work, without enforcing hardware
> >> >> > compartimentalisation (iommu is enforced in software mode)
> >> >> >
> >> >> > When iommu=no-igfx is enforced, shell console boot up works
> flawlessly.
> > All
> >> >> > domu machines get booted up. A system hang will happen at the
> moment a
> > domu
> >> >> > machine does graphic rendering,
> >> >>
> >> >> And this is (other than I originally implied) without passing through
> >> >> the IGD to the DomU? If so, I can't see the difference between a
> >> >> guest rendering to its display (and vncviewer or whatever frontend
> >> >> you use converting this to rendering on the host) and rendering
> >> >> which originates in the host.
> >> >
> >> > Not sure if relevant, but window content is mapped from PV domU
> directly
> >> > into X server (in dom0) address space, using xc_map_foreign_pages. It
> is
> >> > done by hacking XShmAttach function. Not sure what graphics driver do
> >> > with it next. Theoretically it could be possible that driver will
> direct IGD
> >> > to do DMA directly from that place, but I guess it does not.
> >>
> >> Interesting. This then really needs to be investigated from the
> >> Qubes end rather than here. Possible resulting patches, if
> >> relevant outside of that unusual setup, would then of course be
> >> appreciated to be sent here.
> >
>
Interesting fact: kde corrupts video buffer and make system hang faster
then it's xfce counterparts.

> > Note that Thierry said "The point is the iommu=no-igfx doesn't fix the
> > issue", so either it is totally unrelated issue, or iommu=no-igfx is
> > broken. Does iommu=no-igfx have any meaning when IDG is _not_ passed
> > through to some domU?
>
> Yes: Iirc that option turns off the IOMMU responsible for IGD.
>
>  I suppose passing iommu=dom0-passthrough to hypervisor and passing
*intel_iommu*=*igfx_off to dom0 *may be what is desired here, but
hypervisor refuses to boot without iommu=no-igfx flag.

> And generally - is IOMMU used in any way for dom0
> > devices?
>
> Of course; by how much depends on what options you use (see
> the command line doc).
>
Qubes does't use any specifics.

>
> > Is Linux kernel able to utilize it for its own purposes (my
> > guess: no)?
>
> Under Xen? If so - indeed, no.
>
Wouldn't it be more natural if i915 iommu would be deactivated in kernel
with *intel_iommu*=*igfx_off option being passed to dom0?*

>
> > Somehow unrelated: I've tried to get p2m iommu mapping using `xl
> > debug-key o`, but it's too long (and xenconsoled isn't fast enough to
> > catch it into hypervisor.log). Is is possible to enlarge console ring
> > buffer at runtime?
>
> No.
>
> > If not, is `conring_size` the right option?
>
> Yes.
>
> > How large
> > it should be (aprox) for `xl debug-key o` output?
>
> Depends on the amount of memory page tables need to be created
> for as well as how fragmented memory was at the time memory
> allocation for domains happens. I'm afraid you can only try it out.
>
> Jan
>
> Would it be possible to lend a x200 laptop to the most
interested/motivated developer in making i915/iommu work correctly for this
laptop? I don't see how I can troubleshoot this deeper by myself alone.

Thierry

[-- Attachment #1.2: Type: text/html, Size: 5472 bytes --]

[-- Attachment #2: Type: text/plain, Size: 126 bytes --]

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
http://lists.xen.org/xen-devel